text: //===- LoopInvariantCodeMotion.cpp - Code to perform loop fusion-----------===//
  //
  // Copyright 2019 The MLIR Authors.
  //
  // Licensed under the Apache License, Version 2.0 (the "License");
  // you may not use this file except in compliance with the License.
  // You may obtain a copy of the License at
  //
  //   http://www.apache.org/licenses/LICENSE-2.0
  //
  // Unless required by applicable law or agreed to in writing, software
  // distributed under the License is distributed on an "AS IS" BASIS,
  // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  // See the License for the specific language governing permissions and
  // limitations under the License.
  // =============================================================================
  //
  // This file implements loop invariant code motion.
  //
  //===----------------------------------------------------------------------===//
  
  #include "mlir/Analysis/AffineAnalysis.h"
  #include "mlir/Analysis/AffineStructures.h"
  #include "mlir/Analysis/LoopAnalysis.h"
  #include "mlir/Analysis/SliceAnalysis.h"
  #include "mlir/Analysis/Utils.h"
  #include "mlir/Dialect/AffineOps/AffineOps.h"
  #include "mlir/Dialect/StandardOps/Ops.h"
  #include "mlir/IR/AffineExpr.h"
  #include "mlir/IR/AffineMap.h"
  #include "mlir/IR/Builders.h"
  #include "mlir/Pass/Pass.h"
  #include "mlir/Transforms/LoopUtils.h"
  #include "mlir/Transforms/Passes.h"
  #include "mlir/Transforms/Utils.h"
  #include "llvm/ADT/DenseMap.h"
  #include "llvm/ADT/DenseSet.h"
  #include "llvm/ADT/SmallPtrSet.h"
  #include "llvm/Support/CommandLine.h"
  #include "llvm/Support/Debug.h"
  #include "llvm/Support/raw_ostream.h"
  
  #define DEBUG_TYPE "licm"
  
  using namespace mlir;
  
  namespace {
  
  /// Loop invariant code motion (LICM) pass.
  /// TODO(asabne) : The pass is missing zero-trip tests.
  /// TODO(asabne) : Check for the presence of side effects before hoisting.
  struct LoopInvariantCodeMotion : public FunctionPass<LoopInvariantCodeMotion> {
    void runOnFunction() override;
    void runOnAffineForOp(AffineForOp forOp);
  };
  } // end anonymous namespace
  
  static bool
  checkInvarianceOfNestedIfOps(Operation *op, Value *indVar,
                               SmallPtrSetImpl<Operation *> &definedOps,
                               SmallPtrSetImpl<Operation *> &opsToHoist);
  static bool isOpLoopInvariant(Operation &op, Value *indVar,
                                SmallPtrSetImpl<Operation *> &definedOps,
                                SmallPtrSetImpl<Operation *> &opsToHoist);
  
  static bool
  areAllOpsInTheBlockListInvariant(Region &blockList, Value *indVar,
                                   SmallPtrSetImpl<Operation *> &definedOps,
                                   SmallPtrSetImpl<Operation *> &opsToHoist);
  
  static bool isMemRefDereferencingOp(Operation &op) {
    // TODO(asabne): Support DMA Ops.
    if (isa<AffineLoadOp>(op) || isa<AffineStoreOp>(op)) {
      return true;
    }
    return false;
  }
  
  std::unique_ptr<FunctionPassBase> mlir::createLoopInvariantCodeMotionPass() {
    return std::make_unique<LoopInvariantCodeMotion>();
  }
  
  // Returns true if the individual op is loop invariant.
  bool isOpLoopInvariant(Operation &op, Value *indVar,
                         SmallPtrSetImpl<Operation *> &definedOps,
                         SmallPtrSetImpl<Operation *> &opsToHoist) {
    LLVM_DEBUG(llvm::dbgs() << "iterating on op: " << op;);
  
    if (isa<AffineIfOp>(op)) {
      if (!checkInvarianceOfNestedIfOps(&op, indVar, definedOps, opsToHoist)) {
        return false;
      }
    } else if (isa<AffineForOp>(op)) {
      // If the body of a predicated region has a for loop, we don't hoist the
      // 'affine.if'.
      return false;
    } else if (isa<AffineDmaStartOp>(op) || isa<AffineDmaWaitOp>(op)) {
      // TODO(asabne): Support DMA ops.
      return false;
    } else if (!isa<ConstantOp>(op)) {
      if (isMemRefDereferencingOp(op)) {
        Value *memref = isa<AffineLoadOp>(op)
                            ? cast<AffineLoadOp>(op).getMemRef()
                            : cast<AffineStoreOp>(op).getMemRef();
        for (auto *user : memref->getUsers()) {
          // If this memref has a user that is a DMA, give up because these
          // operations write to this memref.
          if (isa<AffineDmaStartOp>(op) || isa<AffineDmaWaitOp>(op)) {
            return false;
          }
          // If the memref used by the load/store is used in a store elsewhere in
          // the loop nest, we do not hoist. Similarly, if the memref used in a
          // load is also being stored too, we do not hoist the load.
          if (isa<AffineStoreOp>(user) ||
              (isa<AffineLoadOp>(user) && isa<AffineStoreOp>(op))) {
            if (&op != user) {
              SmallVector<AffineForOp, 8> userIVs;
              getLoopIVs(*user, &userIVs);
              // Check that userIVs don't contain the for loop around the op.
              if (llvm::is_contained(userIVs, getForInductionVarOwner(indVar))) {
                return false;
              }
            }
          }
        }
      }
  
      // Insert this op in the defined ops list.
      definedOps.insert(&op);
  
      if (op.getNumOperands() == 0 && !isa<AffineTerminatorOp>(op)) {
        LLVM_DEBUG(llvm::dbgs() << "\nNon-constant op with 0 operands\n");
        return false;
      }
      for (unsigned int i = 0; i < op.getNumOperands(); ++i) {
        auto *operandSrc = op.getOperand(i)->getDefiningOp();
  
        LLVM_DEBUG(
            op.getOperand(i)->print(llvm::dbgs() << "\nIterating on operand\n"));
  
        // If the loop IV is the operand, this op isn't loop invariant.
        if (indVar == op.getOperand(i)) {
          LLVM_DEBUG(llvm::dbgs() << "\nLoop IV is the operand\n");
          return false;
        }
  
        if (operandSrc != nullptr) {
          LLVM_DEBUG(llvm::dbgs()
                     << *operandSrc << "\nIterating on operand src\n");
  
          // If the value was defined in the loop (outside of the
          // if/else region), and that operation itself wasn't meant to
          // be hoisted, then mark this operation loop dependent.
          if (definedOps.count(operandSrc) && opsToHoist.count(operandSrc) == 0) {
            return false;
          }
        }
      }
    }
  
    // If no operand was loop variant, mark this op for motion.
    opsToHoist.insert(&op);
    return true;
  }
  
  // Checks if all ops in a region (i.e. list of blocks) are loop invariant.
  bool areAllOpsInTheBlockListInvariant(
      Region &blockList, Value *indVar, SmallPtrSetImpl<Operation *> &definedOps,
      SmallPtrSetImpl<Operation *> &opsToHoist) {
  
    for (auto &b : blockList) {
      for (auto &op : b) {
        if (!isOpLoopInvariant(op, indVar, definedOps, opsToHoist)) {
          return false;
        }
      }
    }
  
    return true;
  }
  
  // Returns true if the affine.if op can be hoisted.
  bool checkInvarianceOfNestedIfOps(Operation *op, Value *indVar,
                                    SmallPtrSetImpl<Operation *> &definedOps,
                                    SmallPtrSetImpl<Operation *> &opsToHoist) {
    assert(isa<AffineIfOp>(op));
    auto ifOp = cast<AffineIfOp>(op);
  
    if (!areAllOpsInTheBlockListInvariant(ifOp.thenRegion(), indVar, definedOps,
                                          opsToHoist)) {
      return false;
    }
  
    if (!areAllOpsInTheBlockListInvariant(ifOp.elseRegion(), indVar, definedOps,
                                          opsToHoist)) {
      return false;
    }
  
    return true;
  }
  
  void LoopInvariantCodeMotion::runOnAffineForOp(AffineForOp forOp) {
    auto *loopBody = forOp.getBody();
    auto *indVar = forOp.getInductionVar();
  
    SmallPtrSet<Operation *, 8> definedOps;
    // This is the place where hoisted instructions would reside.
    OpBuilder b(forOp.getOperation());
  
    SmallPtrSet<Operation *, 8> opsToHoist;
    SmallVector<Operation *, 8> opsToMove;
  
    for (auto &op : *loopBody) {
      // We don't hoist for loops.
      if (!isa<AffineForOp>(op)) {
        if (!isa<AffineTerminatorOp>(op)) {
          if (isOpLoopInvariant(op, indVar, definedOps, opsToHoist)) {
            opsToMove.push_back(&op);
          }
        }
      }
    }
  
    // For all instructions that we found to be invariant, place sequentially
    // right before the for loop.
    for (auto *op : opsToMove) {
      op->moveBefore(forOp);
    }
  
    LLVM_DEBUG(forOp.getOperation()->print(llvm::dbgs() << "Modified loop\n"));
  
    // If the for loop body has a single operation (the terminator), erase it.
    if (forOp.getBody()->getOperations().size() == 1) {
      assert(isa<AffineTerminatorOp>(forOp.getBody()->front()));
      forOp.erase();
    }
  }
  
  void LoopInvariantCodeMotion::runOnFunction() {
    // Walk through all loops in a function in innermost-loop-first order.  This
    // way, we first LICM from the inner loop, and place the ops in
    // the outer loop, which in turn can be further LICM'ed.
    getFunction().walk([&](AffineForOp op) {
      LLVM_DEBUG(op.getOperation()->print(llvm::dbgs() << "\nOriginal loop\n"));
      runOnAffineForOp(op);
    });
  }
  
  static PassRegistration<LoopInvariantCodeMotion>
      pass("affine-loop-invariant-code-motion",
           "Hoist loop invariant instructions outside of the loop");
  